It was first demonstrated in 2007 that CRISPR-Cas is an adaptive immune system in many bacteria and most archaea (Barrangou et al., 2007, Science 315: 1709-1712), Brouns et al., 2008, Science 321: 960-964). Based on functional and structural criteria, three types of CRISPR-Cas systems have so far been characterized, most of which use small RNA molecules as guide to target complementary DNA sequences (Makarova et al., 2011, Nat Rev Microbiol 9: 467-477; Van der Oost et al., 2014, Nat Rev Microbiol 12: 479-492).
In a recent study by the Doudna/Charpentier labs, a thorough characterization of the effector enzyme of the type II CRISPR-Cas system (Cas9) was performed, including demonstration that the introduction of designed CRISPR RNA guides (with specific spacer sequences) targets complementary sequences (protospacers) on a plasmid, causing double strand breaks of this plasmid (Jinek et al., 2012, Science 337: 816-821). Following Jinek et al., 2012, Cas9 is used as a tool for genome editing.
Cas9 has been used to engineer the genomes of a range of eukaryotic cells (e.g. fish, plant, man) (Charpentier and Doudna, 2013, Nature 495: 50-51).
In addition, Cas9 has been used to improve yields of homologous recombination in bacteria by selecting for dedicated recombination events (Jiang et al., 2013, Nature Biotechnol 31: 233-239). To achieve this, a toxic fragment (Targeting construct) is co-transfected with a rescuing fragment carrying the desired alteration (Editing construct, carrying point mutation or deletions). The Targeting construct consists of Cas9 in combination with a design CRISPR and an antibiotic resistance marker, defining the site of the desired recombination on the host chromosome; in the presence of the corresponding antibiotic, integration of the Targeting construct in the host chromosome is selected for. Only when the additional recombination occurs of the Editing construct with the CRISPR target site elsewhere on the host chromosome, the host can escape from the auto-immunity problem. Hence, in the presence of the antibiotic, only the desired (marker-free) mutants are able to survive and grow. A related strategy to select for subsequent removal of the integrated Targeting construct from the chromosome is presented as well, generating a genuine marker free mutant.
It has been established in recent years that CRISPR-Cas mediated genome editing constitutes a useful tool for genetic engineering. It has been established that the prokaryotic CRISPR systems serve their hosts as adaptive immune systems (Jinek et al., 2012, Science 337: 816-821) and can be used for quick and effective genetic engineering (Mali et al., 2013, Nat Methods 10:957-963, for example), requiring only modification of the guide sequence in order to target sequences of interest.
However, there is a continuing need for the development of agents with improved sequence-specific nucleic acid detection, cleavage and manipulation under a variety of experimental conditions for application in the area of genetic research and genome editing. In particular, currently available sequence-specific genome editing tools, including Cas9, are not applicable for use in all conditions or organisms, for example, sequence-specific nucleases are relatively thermo-sensitive and therefore not applicable for use in strictly thermophilic microorganisms (which are capable of growth between 41° C. and 122° C. and grow optimally in the ranges of temperatures from >60° C. to 80° C. with hyperthermophiles capable of optimal growth above 80° C.), for example, microorganisms that are used in industrial fermentations or for in vitro laboratory processes conducted at elevated temperatures.
To date there is no experimental evidence for active Cas9 proteins in thermophiles. Based on a comparative genome screening by Chylinski et al. (2014; Nucleic Acids Research 42: 6091-61-05) on the presence of Cas9 in bacteria it was found that the Type II-C CRISPR-Cas system is only present in approximately 3.3% of all bacterial genomes. Among thermophilic bacteria, the Type II system is underrepresented based on statistical analysis (P=0.0019). In addition, no Type II system has been found in archaea however, this could possibly be due to the absence of the RNase III protein (involved in the Type II system) in archaea. Chylinski, et al., (2014; Nucleic Acids Research 42: 6091-6105) did describe the classification and evolution of type II CRISPR-Cas systems, in particular, two species are identified which exhibit these systems, however these species grow maximally at 55° C. and do not exhibit strictly thermophilic growth with optimum growth temperature 60-80° C., with hyperthermophiles capable of growing optimally above 80° C.
Despite the rarity of the CRISPR-Cas system in bacterial genomes and in particular the fact that Cas9 has been found only in bacteria (not archaea) with optimal growth temperatures below 45° C., the inventors have surprisingly discovered several thermostable Cas9 variants which enable genome editing to be carried out at elevated temperatures. These Cas9 nucleases provide novel tools for genetic engineering at elevated temperatures and are of particular value in the genetic manipulation of thermophilic organisms; particularly microorganisms.